1. Field of the Invention
The present invention relates to alumina particulates, optionally stabilized by doping amounts of rare-earth elements, which retain a high specific surface area when exposed to elevated temperatures. More particularly, the invention relates to a process for preparing stabilized alumina.
2. Prior Art
Transitional aluminas, in particular the xcex3-modification, are extensively used as catalytic supports for automotive gas exhaust catalysts in internal combustion engines due to their high specific surface area. The activity of an alumina-supported catalyst depends on the specific surface area of the alumina. While supports of transitional aluminas, e.g. xcex3-Al2O3, may be used with catalysts to effectively reduce nitrogen oxides and oxidize the carbon monoxide and hydrocarbons contained in gas exhaust, these supported catalysts are unstable when exposed to elevated temperatures, e.g. greater than about 800xc2x0 C. Such elevated temperatures frequently arise in exhaust systems for a significant period of time as a result of fuel detonation in an engine. At temperatures above about 800xc2x0 C., xcex3-Al2O3 rapidly undergoes a phase transition from xcex3-Al2O3 to the thermodynamically stable alpha phase with concomitant drastic decrease in specific surface area and loss of catalytic properties. Additionally this phase transformation is accompanied by sintering, i.e. a particle agglomeration process.
One solution to this problem of thermally unstable transitional alumina is described in U.S. Pat. No. 3,867,312, incorporated herein by reference, which describes doping of transitional alumina with certain rare-earth oxides (particularly lanthania) to delay the undesirable phase transformation and prevent loss of surface area at elevated temperatures. Other dopants have also been used, including barium, cerium, other rare-earth elements, phosphorus, silicon compounds and others. Commercial doping of transitional alumina typically involves the use of La2O3.
Lanthanum doping can be performed by soaking Al2O3 powder in an aqueous solution of a lanthanide salt such as La(NO3)3, La(CH3COO)3 or LaCl3. The lanthanide soaked alumina is subsequently dried at high temperature, e.g. about 600xc2x0 C., followed by annealing. This method of lanthanum doping typically requires the use of about 3.0-3.5 mol % La to achieve a sufficient degree of stabilization. An industrial route involves the dissolution of pure metallic aluminum in hexanol, followed by hydrolysis and thermal treatments to obtain the desired alumina modification. Lanthanum compounds, which are typically used for stabilization, are expensive and add significantly to the cost of preparing stabilized alumina. Accordingly, a need remains for low cost stabilized alumina to reduce the cost of manufacturing catalytic converters.
This need is met by the method of the present invention of preparing thermally stable alumina which retains a high specific surface area after calcination at high temperature and which is suitable, among other applications, for supporting automotive gas exhaust catalysts. The method of the present invention includes steps of a) providing an aqueous solution of an aluminum salt, b) treating the aluminum solution with a hydroxyl group anion-exchanger to produce a composition comprising aluminum hydroxides at a preferred pH of about 6 to about 8, c) freeze-drying the aluminum hydroxide composition to produce an aluminum hydroxide powder and d) dehydrating the aluminum hydroxide powder to yield particulates of xcex3-alumina. The aluminum salt preferably is aluminum nitrate in a 1 molar solution. Dehydration may be achieved by heating the aluminum hydroxide powder to about 600xc2x0 C. to about 800xc2x0 C. followed by a cooling step.
The alumina produced according to the present invention may be further stabilized by including a salt of a lanthanide series element, preferably lanthanum in the form of La(NO3)3, in the aqueous solution. The molar ratio of aluminum to lanthanum in the aqueous solution preferably is about 0.0003 to about 0.03, more preferably about 0.001 to about 0.003. At the more preferred ratio, the final concentration of lanthanum in the xcex3-alumina is about 0.1 to about 0.3 mol %. Transitional alumina produced according to the present invention retains a specific surface area following annealing at about 1000xc2x0 C. for about 3 hours of over 50 m2/g without lanthanum incorporated therein and up to at least 110 m2/g when lanthanum is included in the alumina particles.